I don't think you should just ask her out. If she is already dating someone, she will be put in the akward position of having to say so, knowing it will make you feel akward, too.
I don't know how well you know her, but I think at your next encounter, you should ask if she is seeing anyone before you ask her out. Then she can either say "Yes" and you can smile and say something light hearted like, "I should have known all the best ones are taken" or something cheesy like that.. if she says "No.. I'm not seeing anyone".... then you hand her a pair of glasses... no, just kiddin'... seriously... *then* you ask her....
my deflated 2cents.